1. Field of the Invention
The invention relates generally to tunneling magnetoresistive (TMR) devices, and more particularly to a method for forming a barrier layer consisting essentially of a magnesium oxide (MgO).
2. Description of the Related Art
A tunneling magnetoresistive (TMR) device, also called a magnetic tunneling junction (MTJ) device, is comprised of two ferromagnetic layers separated by a thin insulating tunneling barrier layer. The barrier layer is typically made of a metallic oxide, such as alumina (Al2O3) or MgO, that is so sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the two ferromagnetic layers. This quantum-mechanical tunneling process is electron spin dependent, which means that an electrical resistance measured when applying a sense current across the junction depends on the spin-dependent electronic properties of the ferromagnetic and barrier layers, and is a function of the relative orientation of the magnetizations of the two ferromagnetic layers. The magnetization of the first ferromagnetic layer is designed to be pinned, while the magnetization of the second ferromagnetic layer is designed to be free to rotate in response to external magnetic fields. The relative orientation of their magnetizations varies with the external magnetic field, thus resulting in change in the electrical resistance. The TMR device is usable as a memory cell in a nonvolatile magnetic random access memory (MRAM) array, as described in U.S. Pat. No. 5,640,343, and as TMR read head in a magnetic recording disk drive, as described in U.S. Pat. No. 5,729,410.
FIG. 1 illustrates a cross-sectional view of a conventional TMR device 10. The TMR device 10 includes a bottom “fixed” or “pinned” ferromagnetic (FM) layer 18, an insulating tunneling barrier layer 20, and a top “free” FM layer 32. The TMR device 10 has bottom and top nonmagnetic electrodes or leads 12, 14, respectively, with the bottom nonmagnetic electrode 12 being formed on a suitable substrate. The FM layer 18 is called the “pinned” layer because its magnetization is prevented from rotation in the presence of an applied magnetic field in the desired range of interest for the TMR device, i.e., the magnetic field caused by the write current applied to the memory cell from the read/write circuitry of the MRAM, or the magnetic field from a recorded magnetic layer in a magnetic recording disk. The magnetization of the pinned FM layer 18 can be fixed or pinned by being formed of a high-coercivity film or by being exchange-coupled to an antiferromagnetic “pinning” layer. The pinned FM layer 18 may be replaced by a flux-closure structure, where two ferromagnetic layers are separated by an antiparallel spacer layer and thus antiparallel-coupled to form a flux closure, as described in U.S. Pat. No. 5,465,185. The magnetization of the free FM layer 32 is free to rotate in the presence of the applied magnetic field in the range of interest. In the absence of the applied magnetic field, the magnetizations of the FM layers 18 and 32 are aligned generally parallel in a TMR memory cell and generally perpendicular in a TMR read head. The relative orientation of the magnetizations of the FM layers 18, 32 determines the electrical resistance of the TMR device.
An important read-performance parameter of the TMR device is a high signal-to-noise ratio (SNR). The magnitude of the signal depends on a TMR coefficient (ΔR/R) exhibited by the TMR device, where ΔR or (RMAX−RMIN) is the difference between the resistance measured when the magnetizations of the two ferromagnetic layers are antiparallel (RMAX) and the resistance measured when the magnetizations of the two ferromagnetic layers are parallel (RMIN), and R is RMIN. The magnitude of the noise depends on, in large part, the resistance of the TMR device. Thus to obtain the maximum SNR for a constant power used to sense the TMR device, the resistance of the TMR device must be small and the change in resistance (ΔR) of the TMR device large. The resistance of the TMR device is largely determined by the contact resistances at lower metallic/oxide and upper oxide/metallic interfaces (RC and RC′, respectively) and the resistivity (ρ) of the insulating tunneling barrier layer, which are much larger than the contact resistances at all other metallic/metallic interfaces and the resistivities of all other metallic films, respectively. Moreover, because the sense current flows perpendicularly through all the interfaces, the resistance of the TMR device of a given dimension increases proportionally with the thickness of the barrier layer (δ) and inversely with the area of the barrier layer (A). It is convenient to characterize the resistance of the TMR device by the product of the resistance R times the area A (RA), as a figure of merit to correlate with the amplitude of the noise. As described previously, this RA can be estimated as the sum of RCA, RC′A and ρδ. As the TMR device is further miniaturized to achieve higher densities in memory cells or in recording media, it becomes more stringent to maintain a high SNR. Thus, it is desirable to develop a TMR device with an improved tunneling barrier layer which exhibits a higher ΔR/R and a lower RA.
TMR devices with MgO barrier layers and various processes for forming MgO barrier layers are well known. U.S. Pat. No. 6,841,395 B2 describes a three-step process for making an MgO barrier layer by depositing an Mg film in an argon gas in a DC magnetron sputtering module, depositing an oxygen-doped Mg film in mixed xenon and oxygen gases in an ion-beam sputtering module, and oxidizing these films in an oxygen gas in an oxygen treatment module. US 2007/0111332 A1 describes a process for making a barrier layer comprising Mg/MgO/Mg films wherein the lower and upper Mg films are deposited by a DC sputtering method and the intermediate MgO film is formed by natural oxidation of an Mg film. Tsunekawa et al. “Giant tunneling magnetoresistive effect in low resistance CoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read-head applications”, Appl. Phys. Lett. 87, 072503 (2005) describe a TMR device with an MgO barrier layer comprising Mg/MgO films wherein the Mg film as thin as 0.1 nm is deposited by a DC sputtering method and the MgO film is RF sputter deposited from an MgO target. Although this prior-art process for forming the barrier layer comprising the Mg/MgO films results in a TMR device that exhibits reasonably high ΔR/R and reasonably low RA, the antiferromagnetic pinning and ferromagnetic free layers used in this prior-art process are not suitable for the TMR device to function properly.
What is needed is a method for forming an MgO barrier layer that results in a TMR device exhibiting even higher ΔR/R and even lower RA while maintaining good antiferromagnetic and ferromagnetic properties for the TMR device to function properly.